1. Field of the Invention
This invention relates to an apparatus for the treatment of waste gas containing a combustible compound and a method for the treatment of the waste gas by the use of the apparatus.
2. Description of the Related Art
The waste gas discharged from the process for the production of chemical substances possibly contains combustible compounds such as saturated hydrocarbons like methane, ethane or propane, unsaturated hydrocarbons like ethylene, or propylene, carbon monoxide, organic acids like phthalic acid, maleic acid, or (meth)acrylic acid, aldehydes like formaldehyde, esters, or alcohols in small amounts either singly or in the form of a mixture. These combustible compounds will emit offensive odor even in a small amount. Their release into the atmosphere without treatment is hardly agreeable.
As one such measure, JP-A-57-58012 proposes an apparatus for the treatment of the waste gas. FIG. 1 is a diagram illustrating the apparatus for the treatment of the waste gas, which is disclosed in JP-A-57-58012. In FIG. 1, the apparatus is operated by heating the waste gas supplied through a first heat exchanger 2 and a second heat exchanger 3 disposed in series between a device for supplying the waste gas and a catalytic oxidation reactor 1, supplying part of the hot treated gas emanating from the reactor 1 to the second heat exchanger 3 as the heat source, supplying the treated gas emanating from a heat-recovering device 4 to the first heat exchanger 2 as the heat source, supplying part or the whole of the treated gas used in the second heat exchanger 3 to the reactor 1, and supplying the remainder of the treated gas together with the waste gas emanating from the reactor 1 to the heat-recovering device 4. By allowing the treated gas used in the first heat exchanger 2 to be discharged out of the system, it is possible to maintain the appropriate reaction in temperature with the reactor 1 without addition of unnecessary air. Further by recovering the generated heat utilizing the heat-recovering device 4, it is possible to decrease the heat loss to the outside of the system. That is, since the heat-recovering device 4 absorbs the variation of the heat generated by the reaction due to the conversion of the composition of combustible substances in the waste gas, the temperature of the treated gas emanating from the first heat exchanger is maintained at a substantially fixed level. When the amount of oxygen in the waste gas is not sufficient for the treatment of the combustible component, air may be added in advance into the waste gas to treat it.
In consequence of variations in the operating conditions during the production process, the composition of the waste gas discharged from the process is often changed and this change possibly induces a change in the dew point of the treated gas and causes condensation in the first heat exchanger 2. The waste gas may include a small amount of corrosive substances such as sulfur, which causes corrosion in the heat exchanger due to the condensation. Further, since the dust of the aerated catalyst is scattered from the reactor 1 in the long run, the dust adheres to the portions covered with the condensate possibly to lower the heat exchange ability.
For the purpose of avoiding this situation, it is necessary to elevate the temperature of the treated gas being supplied to the first heat exchanger 2, namely the temperature of the treated gas at the outlet of the heat-recovering device 4. When a waste heat boiler is used for the heat-recovering device 4, for example, the method of decreasing the amount of heat recovered in the boiler and elevating the temperatures of the treated gas at the inlet and the outlet of the first heat exchanger by increasing the steam pressure generated in the boiler, namely elevating the evaporation temperature is conceivable. Since the steam generated in the boiler is generally connected to the steam system in the plant, it is commendable for the pressure of the steam supplied to the steam system to be maintained at a fixed level. For this reason, it is extremely difficult to adjust the temperature of the treated gas at an arbitrary level by varying the steam pressure generated in the waste heat boiler in accordance with the operation condition of the apparatus for the treatment of the waste gas. As a result, the waste gas subjected to the treatment is limited in a narrow range and the apparatus possibly fails to apply to the treatment of the waste gas discharged from the production process.